OpenJDK : Java Glossary

Sun has released much of the Java version 1.6 as open source.
It includes:

Javac.exe, the Java compiler

most of the class libraries

They have not released:

4% of the class library code because some of the class library code is
encumbered meaning that Sun can’t release it under
the GPL (Gnu Public Licence), presumably because they licenced it from someone
else. The encumbrances are in the sound system, font rasterizer, graphics
rasterizer, SNMP (Simple Network Management Protocol) and
JCE (Java Cryptography Extension) areas.

Sun previously allowed you to look at the code if you signed an agreement that
many programmers such as myself were unwilling to sign. With OpenJDK there will be
many more eyes looking over the code for bugs and security leaks.